1. Field of the Invention
The present invention relates to a track retrieving method for making a light beam instantaneously jump and scan from one track to another, thereby reading/writing information from/into a disk having the tracks of a spiral shape or a concentric circle shape. The present invention relates to a method for detecting a middle point between the tracks, the method being required for the track retrieving method, and to an apparatus for use in the methods. In particular, the present invention relates to a jumping scanning method.
2. Description of the Related Art
Several kinds of optical reading/writing apparatuses have been conventionally proposed. Examples of an information medium for use in the optical recording/reproducing apparatus include a disk in which a recording film made of a material capable of optically writing and reading data is deposited by a vapor deposition or the like on a substrate having concentric-circle like tracks in a concave and convex structure. In the optical writing/reading apparatus, a light beam emitted from a light source of a semiconductor laser or the like is irradiated onto such a disk, thereby recording and reproducing data. More particularly, for example, in the case of writing data, the power of the light beam is strongly or weakly modulated in accordance with the data, thereby recording the data by changing the reflectivity of the recording film of the disk. In the case of reproducing data, the power of the light beam is set to be uniform and comparatively weak, thereby reading the data by changing the power of light reflected from the disk.
Such an optical recording/reproducing apparatus performs a focus control for controlling the light beam so as to be always in a schematically predetermined convergent state on the recording film, and a tracking control for controlling the light beam so as to always correctly scan a predetermined track. Furthermore, as is described in detail in Japanese Patent Application No. 63-298717, the apparatus also performs a jumping scan for shifting the light beam from one track to its adjacent one.
The jumping scan is usually performed by means of a tracking shift mechanism applied for the tracking control. The tracking shift mechanism makes the light beam shift relatively with respect to the track in a direction perpendicular to the stretcher direction of the track and horizontal to the disk surface (i.e., in the direction of the disk radius).
The operation of the conventional jumping scan, when applied from a track to its adjacent one on the side of an internal circumference of the disk, will be described as follows with reference to FIGS. 13 and 14.
FIG. 13 is a timing chart for the jumping scan. In FIG. 13, (a) is the timing chart of a tracking control ON/OFF signal; (b) is the timing chart of a drive signal for the tracking shift mechanism; (c) is the timing chart of the tracking error signal; (d) is the timing chart of a binary signal of the tracking error signal; and (e) is the timing chart of a track middle-point detection signal for detecting a middle point between adjacent tracks. In FIG. 14, (a) shows a positional relationship between a light beam 1 and a track 2 on the disk surface; and (b) is a plot of the tracking error signal corresponding to respective positions of the light beam 1.
First, the tracking control ON/OFF signal is set at an OFF level at the timing of T1, as shown in FIG. 13 (a), thereby making the tracking control non-operational. Simultaneously, a rectangular-shaped acceleration pulse is applied to the tracking shift mechanism, as shown in FIG. 13 (b), thereby making the light beam accelerated to shift toward a target track. After the acceleration pulse is completed, a drive signal to the tracking shift mechanism is set at zero level, so that the light beam is shifted by the inertia. At the time when the light beam reaches nearly the middle point between adjacent tracks, that is, at the timing of T2, a deceleration pulse is applied to the tracking shift mechanism, thereby decelerating the light beam. The deceleration pulse has a reverse polarity, although it has the same rectangular shape as that of the acceleration pulse. After the deceleration pulse is completed, the tracking control ON/OFF signal is set as an ON level at the timing of T3, thereby again operating the tracking control. Because of the tracking control, the light beam is pulled into the adjacent track, thereby completing the jumping scan.
In the above jumping scan, it is important to detect with high accuracy the middle point between adjacent tracks, i.e., a track middle point, in order to generate the deceleration pulse at an appropriate timing. The detection of the track middle point will be described as follows:
The detection of the track middle point is performed by detecting an edge of a binary signal of the tracking error signal, after the acceleration pulse is completed. The tracking error signal shows a positional relative relationship between the track and light beam. It is known that the tracking error signal is detected by a push-pull method, with respect to a track having a concave and convex structure with an optical depth of .lambda./8 (.lambda.:a wavelength of the light beam). As is shown in FIG. 14 (b), the tracking error signal draws a sine wave in accordance with the positional relative relationship between the light beam 1 and track 2. The cycle of the sine wave is equal to a track pitch. When the light beam 1 is on a track center, the amplitude of the sine wave is at zero level. In the case of performing the jumping scan, the polarity of the tracking error signal is inverted with respect to the track center as shown in FIG. 13 (c), and the binary signal of the tracking error signal becomes as shown in FIG. 13 (d). Therefore, in the jumping scan toward an outer circumference direction of the disk, the detection of the track middle point is implemented by detecting a rise edge, after the acceleration pulse is completed. That is, a fall edge of the binary signal of the tracking error signal is detected, after the acceleration pulse is completed, thereby generating a track middle-point detection signal as shown in FIG. 13 (e).
In recent years, several kinds of high density disk formats have been proposed for the purpose of increasing a capacity of an optical disk. As an example, an optical disk of a land/groove format (hereinafter, referred to as "an L/G disk") is included. FIG. 15 shows an external appearance view of an L/G disk. The L/G disk will be described below with reference to FIG. 15.
The conventional optical disk has used, as a track, either a groove for the tracking control, or a land which is an intermediate region between the grooves, thereby recording information. On the contrary, the L/G disk uses both lands and grooves as the track, for recording data thereon. Accordingly, the track pitch becomes equal to 1/2 of the conventional one. Thus, the capacity of the optical disk can be made double-fold. In the L/G disk, groove tracks (i.e., hunting portions in FIG. 15) using the groove and land tracks (i.e., portions each sandwiched by the groove tracks) using the land are alternatively coupled to each other for each rotation. As a result, the groove and land tracks form a single spiral shape, whereby a continuous recording or reproduction of data can be made possible without any interruption on the entire disk. Moreover, respective tracks have a corresponding address section. As a result, the tracks can be distinguished from one another. The address sections are matched to one another in a circumference direction of the disk and each provided a forward corresponding land-groove transition section. Such an L/G disk is referred to as a one-spiral L/G disk, since the tracks form a single spiral shape on the disk.
FIG. 16 is an enlarged view of a circled portion of FIG. 15 including the address sections and land-groove transition sections. The transition sections are provided for each rotation of the disk and matched to one another in the circumference direction regardless of the radius direction of the track. Each groove is configured to a convex structure so that its optical length d satisfies the relationship of d=8/ .lambda. with respect to the wavelength .lambda.. Each land track is a flat portion without any grooves. Therefore, when moving on the single spiral-shaped track, the light beam will alternatively be on the groove track and the land track alternatively for each rotation. Addresses of each address section of the convex structure are formed as a pit series. It is known that by the power of the reflected light varied by the pit series, the address section on which the light beam is located can be read. The pit width of the respective addresses is thinner than the track width and the pit length is approximately in the range of from a radius to a diameter of the light beam.
FIG. 17 shows an external appearance view of an L/G disk of a type different from that of FIGS. 15 and 16. The L/G disk of FIG. 17 has land tracks and groove tracks adjacent to each other. The land tracks and the groove tracks respectively form a one-spiral shape. Accordingly, the land-groove transition section is not provided for each track, as is different from that of the L/G disk in FIG. 15. Such an L/G disk is referred to as a two-spiral L/G disk, since there exists two spirals of the lands and grooves, on the disk.
Even in this L/G disk, the land and groove tracks are alternatively provided in the radius direction of the disk, so that the polarity of the tracking control is inverted.
It is not necessary in the two-spiral L/G disk to switch the polarity of the tracking control for each track, in the case of scanning along the spiral. However, it is not possible to realize a continuous writing or reading of data on the entire disk. That is to say, for example, after data is written and read from the outer circumference to the internal circumference along the spiral of the groove track, the light beam should be again shifted to the most outer circumference, thereby writing and reading data in the spiral of the land track. Other structural characteristics of the two-spiral L/G disk are the same as those of the single spiral L/G disk.
FIG. 18 shows the positional relationship between the light beam 1 and tracks 2 on the above L/G disk, and the corresponding tracking error signal in a horizontal axis. In the L/G disk, the land region between the groove tracks which correspond to the conventional track becomes the land track. With respect to the groove tracks, the tracking error signal is exactly the same as that of the conventional disk. That is, when the light beam is in the center of the groove track, the tracking error signal draws the sine wave whose amplitude becomes a zero level, the cycle thereof being equal to the pitch of the groove track. The land track is provided at a position where a phase of the tracking error signal shifts a 180.degree. with respect to the groove track. Therefore, the polarity of the tracking error signal is inverted between the groove rack and land track.
As is mentioned above, the detection of the track middle point, which is indispensable for the jumping scan, is performed by detecting the edge of the binary signal of the tracking error signal. However, in the L/G disk, the edge does not correspond to the middle point between the groove track and land track. Therefore, the track middle point cannot be detected by the conventional method, so that the jumping scan cannot be performed.
The above is the same to a simple servo format disk of an inverted wobble system, to which the conventional method cannot be applied in order to detect the track middle point.